1. Technical Field
The present invention relates to a contraction type actuator in which a magnetostrictive rod contracts in an axial direction by applying a magnetic field to the magnetostrictive rod. In particular, the present invention relates to a contraction type actuator which can efficiently and evenly apply a bias magnetic field to the magnetostrictive rod to obtain a large contraction amount, while having a small and simple structure.
2. Background Art
Conventionally, an actuator which utilizes the characteristic of a magnetostrictive element in which applying a magnetic field distorts a magnetic material is widely known. In this magnetostrictive element, a magnetostrictive member can expand and contract by controlling the strength of a magnetic field applied to the magnetostrictive member. The actuator using such a magnetostrictive element is widely applied to the field of precision equipment, a fuel injection valve of an automobile, and the like.
For example, Japanese Patent Laid-Open Publication No. Hei 11-22587 discloses a contraction type actuator. In the contraction type actuator, a cylindrical magnetostrictive rod is expanded in advance by applying a bias magnetic field to the magnetostrictive rod, and then the magnetostrictive rod contracts (carries out contraction operation) by applying a magnetic field in the opposite direction of the bias magnetic field thereto by means of a solenoid.
By the way, a magnetostrictive rod applied to such a contraction type actuator generally takes the shape of a slim cylinder in order to increase a contraction amount (displacement amount), miniaturize the contraction type actuator, prevent the occurrence of a diamagnetic field due to its shape, and the like. This cylindrical magnetostrictive rod is disposed in the inner space of a bias magnet in an approximately cylindrical shape, to apply a bias magnetic field in a certain direction to the magnetostrictive rod.
In the conventionally disclosed contraction type actuator, however, since the bias magnet has the approximately cylindrical shape, a bias magnetic field applied (emitted) to the outer space of the bias magnet is stronger than the bias magnetic field applied to the inner space, in which the magnetostrictive rod is disposed. Thus, there was a problem that the bias magnetic field could not be efficiently applied to the magnetostrictive rod.
Since the magnetostrictive rod, to which the bias magnetic field is applied, takes the shape of a slim cylinder, the vicinity of the center of the cylinder in an axial direction is far from a magnetic pole of the bias magnet. As a result, the strength of the bias magnetic field in the vicinity of the center in the axial direction is weaker than those in the vicinity of both ends in the axial direction. Thus, there was a problem that the strength of the applied bias magnetic field varied in accordance with the axial position of the magnetostrictive rod. Furthermore, such problems became obvious with increasing the length of an axis of the magnetostrictive rod to increase the contraction amount.